The invention relates to communications in general. More particularly, the invention relates to a method and apparatus to automatically perform gain equalization in an optical transmission system utilizing a plurality of optical amplifiers such as raman amplifiers.
Optical fiber amplifiers are fundamentally important to long-haul optical communications systems. Optical signals begin to attenuate as they travel over an optical fiber transmission medium due to a variety of factors such as fiber loss and dispersion. Optical amplifiers help compensate for such attenuation by providing additional power to the optical signal as it moves through the system.
There are two general classes of optical amplifiers. The first class of optical amplifiers is referred to as lumped amplifiers. Lumped amplifiers linearly increase optical signal power of a supplied input signal via stimulated emission of fiber dopants such as erbium that is subject to an optical pump source. An example of a lumped amplifier would be an Erbium Doped Fiber Amplifier (EDFA). The second class of optical amplifiers is referred to as distributed amplifiers. Distributed amplifiers increase optical signal power along the signal transmission path. An example of a distributed amplifier is a raman amplifier. Although each class of amplifiers has its own advantages and disadvantages, distributed amplifiers in general, and raman amplifiers in particular, are highly desirable since they offer potentially lower noise levels and higher potential bandwidth than lumped amplifiers.
Raman amplification is accomplished by introducing the signal and pump energies along the same optical fiber. The pump and signal may be copropagating or counterpropagating with respect to one another. A raman amplifier uses Stimulated Raman Scattering (SRS), which occurs in silica fibers when an intense pump beam propagates through it. SRS is an inelastic scattering process in which an incident pump photon loses its energy to create another photon of reduced energy at a lower frequency. The remaining energy is absorbed by the fiber medium in the form of molecular vibrations (i.e., optical phonons). That is, pump energy of a given wavelength amplifies a signal at a longer wavelength.
FIG. 1 (PRIOR ART) is a plot illustrating the relationship between the pump energy and the raman gain for a silica fiber. The particular wavelength of the pump energy that is used in this example is denoted by reference numeral 1. As shown in FIG. 1, the gain spectrum 2 for this particular pump wavelength is shifted in wavelength with respect to the pump wavelength. Consequently, the bandwidth of the raman amplifier is limited. For example, the bandwidth of the amplifier shown in FIG. 1 is only about 20 nanometers (nm) at a gain of 10 decibels (dB).
FIG. 2 (PRIOR ART) is a plot illustrating the relationship between the pump energy from multiple pumps and the raman gain for a silica fiber. One technique to increase the bandwidth of a raman amplifier is through the use of multiple pumps operating at different wavelengths. As shown in FIG. 2, pump energy supplied at a wavelength denoted by reference numeral 202 generates a gain curve 204 while pump energy supplied at a wavelength denoted by reference numeral 206 generates gain curve 208. The composite gain spectrum, indicated by curve 210, has a bandwidth that is greater than either of the individual gain curves 204 and 208.
One problem associated with a raman amplifier using multiple pumps, however, is that the gain curve for each pump is shifted with respect to adjacent gain curves. This result is due to the use of different wavelengths for each pump. Consequently, the different pump wavelengths generate gain curves having different gain maxima thereby creating an uneven gain signal. This uneven gain signal is sometimes referred to as a xe2x80x9cgain ripple.xe2x80x9d
Conventional systems attempt to compensate for the gain ripple using a variety of techniques. These techniques, however, are unsatisfactory for a number of reasons. For example, one technique is to use a multistage amplifier wherein each stage uses different fiber lengths, different dopants, fibers of different geometry, or pumping power. Each of these factors, however, is imprecise in terms of controlling the amount of gain per stage and therefore the composite gain signal. The technique of varying gain curves by varying the type or quantity of dopants is particularly complex and inaccurate. Furthermore, multistage amplifiers are inherently more complex and expensive to build and deploy in a lightwave system. Another technique is to utilize an equalizer or optical filter for each amplifier. This technique, however, requires additional components and therefore adds complexity and expense to the system.
In view of the foregoing, it can be appreciated that a substantial need exists for a method and apparatus that solves the above-discussed problems.
The embodiments of the invention include a method and apparatus to perform automatic gain equalization for an optical communications system. A first amplifier amplifies an optical signal to produce a first gain curve having a first gain maxima and a first gain minima of a first period. A second amplifier amplifies the optical signal to produce a second gain curve with a second gain maxima and a second gain minima of a second period, with the second gain maxima substantially coinciding with the first gain minima.
With these and other advantages and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several drawings attached herein.